Recently, multilayer structure has been widely employed in printed wiring boards for miniaturization of the printed wiring boards by increased wiring density of printed wiring boards. Multilayer printed wiring boards are used in many mobile electronic devices to achieve downsizing. Then, multilayer printed wiring boards are required both further thickness reduction in interlayer insulating layers and further weight reduction.
As a technology which satisfies such requirements, a manufacturing method employing the coreless build-up method have been used. In the coreless build-up method, a copper foil with carrier foil has been utilized for separation between the supporting substrate and the multilayer printed wiring board. As the technologies relating to the coreless build-up method, the following technologies exist.
Patent Document 1 discloses followings as can be seen in the drawings of Patent Document 1 for the purpose of improving a packaging substrate and the manufacturing method: a core substrate has construction wherein carrier copper foils 3 of copper clad laminates are stacked on both sides of a pre-preg 2. The core substrate is made to be a first circuit board 10. Wiring conductor is formed on an ultra-thin copper foil 4, and a second circuit board 20 is made. An insulated resin layer is formed on the wiring conductor, and a third circuit board 30 is made. A conformal mask is formed, and a fourth circuit board 40 is made. A blind hole is formed, and a fifth circuit board 50 is made. Copper plating in the blind hole is carried out, and the hole is electrically connected. Wiring is etched on the blind hole, and a sixth circuit board is made. A supporting substrate including a carrier copper foil is removed, and a seventh circuit board is made. The ultra-thin copper foil is removed, and an eighth circuit board is obtained. In this manufacturing method, the matter is disclosed that an ultra-thin copper foil with a carrier copper foil is used, and a build-up layer is formed on the surface of the ultra-thin copper foil.
Next in Patent Document 2, the following composite copper foil is disclosed to provide a composite copper foil in which fluctuation in release strength due to the heating temperature is small, a support copper foil is easily releasable after the lamination on a resin substrate and release strength is stable: the composite copper foil consists of a thermal diffusion preventing layer which hinders the diffusion of copper between the support copper foil and an ultra-thin copper foil due to heat and a release layer which mechanically separates the support copper foil and the ultra-thin copper foil provided between the support copper foil and the ultra-thin copper foil. Further, a thermal diffusion preventing layer is described as follows in the column 0007 in Description of Patent Document 2: when a Ni—P alloy layer is used as a thermal diffusion preventing layer, preferable thickness is 0.01 to 5 μm and more preferable thickness is 0.05 to 1 μm; when the thickness is less than 0.01 μm, pinholes may exist, and release strength tends to be made unstable; and when the thickness exceeds 5 μm, the productivity tends to be made poor. According to Examples in Patent Document 2, 0.1 μm is the thinnest heat-resistant metal layer used.